The probe-forming part of an electron beam tomograph known from D. P. Boyd et al., U.S. Pat. No. 4,352,021, and K. R. Peschmann et al., Applied Optics 24 (No. 23), December 1985, pp. 4052-4060 is essentially composed of an electron gun, of an evacuated drift tube that expands step-by-step and of a solenoid lens that deflects the electrons out of the beam axis and focuses them on an anode ring serving as an x-ray source. The electron beam should spread as far as possible due to its space charge between the electron gun and the solenoid lens since the size of the electron focus on the anode ring, and thus the size of the x-ray source as well, is inversely proportional to the cross section of the beam entering the solenoid lens. As a consequence of the interaction of the electrons with the residual gasses that are always present in the drift tube, however, positive ions arise that at least partially neutralize the space charge of the beam, and thus reduce the widening thereof.
For resolving this problem, R. E. Rand et al., U.S. Pat. No. 4,521,900, and R. E. Rand et al., U.S. Pat. No. 4,625,150 proposes that the gas ions produced by electron impact be removed from the beam with the assistance of electrical or magnetic deflection systems. The electrostatic deflection system described in R. E. Rand et al., U.S. Pat. No. 4,625,150 is a matter of an improved version of the electrode system known from R. E. Rand et al., U.S. Pat. No. 4,521,900. It makes it possible to generate more uniform electrostatic cross-fields and avoids the deflection of the electron beam. The two anti-parallel, electrical cross-fields, however, effect a dislocation of the electron beam so that it enters the solenoid lens off-axially. This increases the operations of the system (off-axial astigmatism) and complicates the control of the position of the electron focus on the anode ring.